And tonight, killer whales kept in captivity. Tourists love the see them jump through the hoops and leap out of the water. Tonight, abc's linsey davis tells us about the new documentary that re-examines an incident at sea world that killed a trainer. It raises new questions about the creatures in captivity. Reporter: A seemingly playful scene that turns deadly in an instant. Three years ago, tilikum, a 12,000 pound 22 foot long killer whale killed 40-year-old seaworld trainer dawn brancheau. His story is now the subject of a new documentary called "blackfish." I didn't understand why a killer whale would essentially bite the hand that feeds it. Reporter: In her documentary, gabriela cowperwaite questions, should killer whales be kept in captivity? Is it true that killer whales have never been responsible for loss of life in the wild? This is true. There's no documented case of a killer whale ever killing anybody in the wild. It's only in captivity. Reporter: Cowperthwaite goes back 40 years, when the first killer whales were captured for marine parks. They had speedboats. They had bombs they were throwing in the water to herd the whales into coves. Reporter: Tilikum was about 2 years old when he was captured off the coast of iceland in 1983. A year later, he was sent to marine park where he and two other killer whales were kept at night in a holding pen just 20 feet wide and 30 feet deep. Closing that door on him and knowing that he's locked in there for the whole night is like -- it's a stab. It's a, whoa. Reporter: During tilikum's 30 years in captivity he has been associated with 3 of the 4 deaths involving killer whales in parks. Seaworld says that, "since 2010 the company has voluntarily implemented significant changes to the training protocols for its killer whale program that have proven to be safe and effective." While all killer whales are now separated from trainers in shows, tilikum remains at seaworld, as the search for answers continues. Linsey davis, abc news, new york city. Again, linsey davis reporting on a new documentary.

This transcript has been automatically generated and may not be 100% accurate.

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